The present invention relates to a switching power supply or converter. More particularly, the present invention relates to such a switching power supply wherein a regulating voltage for the power supply is generated on the secondary side of the circuit and is transferred to a control circuit on the primary side of the circuit so as to stabilize or regulate the dc or operating voltages generated on the secondary side.
In a prior art switching power supply, a regulating voltage is derived from the pulse voltage across the transformer or from the operating voltage generated on the secondary side of the transformer, and thus of the power supply circuit. This regulating voltage controls a control circuit for a switching transistor provided on the primary side in the sense of a pulse width modulation, so that the pulses which control the switching transistor to be conductive are modulated and the operating voltages generated on the secondary side are stabilized.
It is also known to obtain a regulating voltage on the secondary side from a generated operating voltage, to convert this regulating voltage into a pulsed voltage which has been amplitude modulated with the regulating voltage, to transfer the amplitude modulated pulsed voltage to the primary side via a transformer, and to there reconvert the modulated pulsed voltages, by means of a rectifier, back to a regulating voltage which is then fed to the control input terminal of the control circuit.
If such a regulating circuit malfunctions, e.g. due to an interruption in the line or a faulty component, there exists the danger that the switching transistor provided on the primary side is controlled to be conductive for too long a period of time and excess voltages result on the secondary side.